An indoor distribution system of modern cellular mobile communication network widely employs ceiling-mount omnidirectional antennas. Amount of the ceiling-mount omnidirectional antennas accounts for more than 95% of antennas for an indoor distribution system. Technique requirements of the existing standard for ceiling-mount omnidirectional antenna includes: ranges of frequency are 806˜960 MHz and 1710˜2500 MHz; voltage standing wave ratio (VSWR) is <1.5; the gain is 2 dBi in low frequency band, and is 5 dBi in high frequency band.
A basic principle of a ceiling-mount omnidirectional antenna is half wavelength dipole antenna. A ceiling mount omnidirectional antenna usually consists of a monopole and a reflecting plate. The monopole may have microstrip patch of cone shape, column shape, ball shape, square shape, butterfly shape or various combinations or modifications thereof, various shapes and so on. To thicken or broaden a dipole may increase the working bandwidth; the reflecting plate is generally flat plate of circle shape, oval shape or square or flat plate with cone roof. The reflecting plate is equivalent to another arm of dipole antenna. On one hand, it forms a mirror image of monopole and reflects electric waves at the same time so as to strengthen radiation at the side of monopole. The higher the frequency, the stronger the reflection, and the closer the feed point to the reflecting plate, the stronger the reflection. On the other hand, it is convenient for a mounting on indoor ceiling and reducing a protrusive height of an antenna so as to minimize the impact on indoor circumstance. The mainstream product of conventional ceiling-mount antenna is a structure of a combination of single cone and reflecting plate, while some products with relative poor quality are a double-cone structure.
The existing ceiling-mount omnidirectional antennae are originally designed for signal indoor coverage of mobile communication wireless networks which work at low frequency band 806˜960 MHz, such as GSM 900 and CDMA. At this frequency band, a ceiling-mount omnidirectional antenna is characterized as a normal symmetrical half wavelength dipole. In spherical coordinates with Z axis perpendicular to the ground when the antenna mounted on ceil as shown in FIG. 1a and FIG. 1b, the typical radiation pattern in equatorial plane (also called as a horizontal plane, H plane) being a circle; and in meridian plane (also called as a vertical plane, E plane) is a “∞” shape, the maximum gain is at the direction about θ=90°. The antenna gain is about 2 dBi. Except in the range of a small angle near Z axis direction (θ<30°), the difference of the gain with the direction variation is not obvious (less than 3 dB). In high frequency band (1710˜2500 MHz), the radiation pattern in equatorial plane is a circle; whereas in meridian plane is a bilobed lung shape. It behaves directional obviously and the maximum gain being at about θ=35°, greatly different from that at low frequency band even though the antenna gain is about 5 dBi, higher than that at low frequency band (see FIG. 1a and FIG. 1b).
The strong directivity of existing ceiling-mount omnidirectional antenna at high frequency band is determined by length of the dipole and reflecting characteristics of electromagnetic wave. As for high frequency band, the equivalent length of dipole is longer than one wavelength, the main radiation lobe split in a “*” shape. In addition, for a ceiling-mount antenna, if the reflecting plate has a relative large size, the reflecting effect is stronger.
In a test of existing ceiling-mount omnidirectional antennae, the result demonstrates that, the antenna gain is stable with a slight change when the radiating angle θ>60° in low frequency band (806˜960 MHz) (see FIG. 1a); but radiation focuses towards right under the antenna in high frequency band (1710˜2500 MHz), and the maximum gain direction in meridian plane is θ≈35°, the gain attenuating about 3 dB when θ=60°, about 7 dB when θ=80°, about 8 dB when θ=85°. FIG. 1a and FIG. 1b are radiation patterns in E plane at frequency of 800 MHz and 2170 MHz respectively, which are two typical radiation patterns at higher and low frequencies, reflecting basic features of radiation patterns in higher and low frequency bands. As can be seen, the antenna gain attenuates rapidly as the radiating angle θ increases from 60° to 85°.
In high frequency band, the gain's rapid attenuation with the radiation angle increase is a crucial technical defect of the existing ceiling-mount omnidirectional antennae. It causes the energy of mobile communication wireless signals, such as DCS1800 and 3G networks, in indoor distribution system to focus right under the antenna excessively, viz. focus within the radiating angle θ less than 60°. Therefore, the signals strength attenuates rapidly with distance, the effective coverage radius is small and the coverage efficiency is low, thereby it reduces the effect of whole indoor distribution system.
Another defect of the existing ceiling-mount omnidirectional antennae are their high un-roundness of H-plane radiation pattern. As the mainstream products are small in size and impedance in low frequency is not matched. It is necessary connect a metal sheet (or line) to adjust impedance. In addition, in accordance with Standard GB T 21195-2007, directly grounding is required for lightening prevention, and the impedance matched sheet also plays the role for monopole grounding. However, the impedance matched sheet spoils axis symmetry, rendering poor uniformity with azimuth and high un-roundness of H plane radiation pattern. A kind of the existing ceiling-mount omnidirectional antennae with better quality adopts three impedance matched sheets, while most of them with poor quality only adopt a single impedance matched sheet. So the omnidirectional antennae behave as directional ones obviously because of the impedance sheet(s). In high frequency band, at a high radiating angle θ (85° typically) corresponding to an antenna's coverage edge, the un-roundness of H plane radiation pattern for three impedance matched sheets is generally 1.5˜3 dB, equivalent to the difference between the maximum and the minimum gain of 3˜6 dB; for a single impedance matched sheet is generally 3˜6 dB, equivalent to a difference between the maximum and the minimum gain of 6˜12 dB.
A real application scene is provided as follow for further explanation the problems caused by the above technical defects of the existing ceiling-mount omnidirectional antennae.
The interior floor of a common building has a height of about 3 m. No matter whether a mobile user stands or sits at a desk, a mobile communication terminal is usually above shoulders, so the height of mobile communication terminal off the floor is generally higher than 1 meter, and the height between indoor ceiling-mount antenna and mobile communication terminal is less than 2 meters. In indoor distribution system design principle, an antenna coverage radius is: less than 10 m for dense and important building, about 15 m for common building or 20 m for open region. As can be seen by calculation, the radiating angle θ to the above antenna coverage edge is 79°, 82° or 84° respectively. So 85° can be the typical radiating angle to antenna coverage edge. In accordance with FIG. 1a and FIG. 1b, at this angle, the gain of the existing ceiling-mount omnidirectional antenna attenuates 7˜8 dB. If the maximum gain 5 dBi, the antenna gains at these angles are only −2˜−3 dBi. But in the region of radiating angle θ≦60°, the gain is relatively high (less than 3 dB attenuation), and the coverage radius is less than 3.5 m.
It can be concluded that the existing ceiling-mount omnidirectional antenna causes DCS1800 and 3G signals to mainly focus within a range of 3.5 m coverage radius, and the large portion of the designed coverage region, radius from 3.5 m to the edge, the antenna gain attenuates up to 7˜8 dB, together with path loss increasing by frequency and distance. The coverage radiuses of DCS18000 and 3G signals are much smaller than that of GSM 800 MHz, so all these signals coverage cannot be synchronous.
In order to obtain a better indoor signal coverage, it just can raise antenna input power or increase density of antenna layout. But the antenna input power is limited by meeting hygienic standard for environmental electromagnetic waves and the minimum coupling loss (MCL) (In 3G networks, the input CPICH power to the existing ceiling-mount omnidirectional antenna should be less than 5 dBm). Therefore, “low power, abundant antennas” as 3G indoor distribution system design principle is generally adopted, and a scale of indoor distribution system construction and reconstruction are multiplied, thereby bringing about the enormous investment for 3G indoor distribution system construction and reconstruction.
High un-roundness of existing ceiling-mount omnidirectional antennae renders the signal covering not uniform and stable. On the same radius circle, strengths of signals change with azimuth, showing obviously directional. Through the above calculation, on the coverage edge, the difference of signal strength is 2˜4 times for a single impedance matched sheet antenna, while 4˜more than 10 times for 3 impedance matched sheets antenna, rendering the signal coverage deficient in some places and yet excessive in some other places, which reduces network quality.
Besides that, as 2G and 3G signals cover asynchronously, adding more antennas to satisfy 3G signal covering causes 2G signal too strong and power waste and results in more serious signal outdoor leakage, reducing 2G network quality and efficiency. The increase of antennas also brings about more power distribution loss, which wasting more signal power.
Therefore, a principle of “low power, abundant antennas” is forced to be adopted due to the uneven indoor distribution of 3G signal. Moreover, the purpose of this principle is to obtain a quality of 3G network at the expense of increasing investment cost and sacrificing 2G network qualities.
In an indoor distribution system, the more uniform the signal distribution within the target covering region, the better, while the weaker the signals outside the target region, the better. But point source of electromagnetic wave radiates over spherical surface. In a free space, signal energy reduces according to square of propagation distance, that is, 6 dB losses for double range. There is the strongest signal strength under antenna, and the closer to the antenna, the quicker the signal strength attenuates, while the farther the signal away from the antenna, the slower the signal strength attenuates. So, the signal coverage of an indoor ceiling-mount omnidirectional antenna mainly focuses on factors such as the maximum permitted input power, the minimum signal strength at coverage edge, uniformity and stability of signal within coverage and so on.